CSC209 - Software Tools and System Programming

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1 CSC209 - Software Tools and System Programming Fall, 2016 Lab 6 Stathis Maneas smaneas@cs.toronto.edu October 28, 2016

2 Miscellaneous Room: BA 3175 Fridays: 10:00-11:00 am Lecture Duration: ~20 Questions: 40 Download this lecture from: Ask me as many questions as possible! Send me s with your questions!

3 Course Resources Course website: Course forum: PCRS: MarkUs:

4 Valgrind An instrumentation framework for building dynamic analysis tools. Automatic detection of memory management bugs. Threading bugs are detected as well, but for now, we will focus on memory management only. Use the -g compilation flag in order to enable debug symbols.

5 Valgrind - Memcheck A memory error detector tool (enabled by default). Specify the --tool=memcheck flag on the Valgrind command line. It can be used to detect the following problems (among others): Usage of uninitialized variables. Reading/Writing memory that has already been freed. Reading/Writing after the end of a malloc d block. Incorrect freeing of heap memory. Memory leaks.

6 Valgrind - Uninitialized Variables (1/7) The C compiler does not initialize any declared variable. A very common source of errors: Dereferencing uninitialized pointers. Uninitialized accumulators and counters, etc.

7 Valgrind - Uninitialized Variables (2/7) #include <stdio.h> int main() { int x; printf ("x = %d\n", x); return 0; } What s the problem in this code snippet?

8 Valgrind - Uninitialized Variables (3/7) #include <stdio.h> int main() { int x; printf ("x = %d\n", x); return 0; } wolf:~/lab6$ gcc -Wall -g -std=c99 -o lab6 lab6.c lab6.c: In function 'main': lab6.c:4:9: warning: 'x' is used uninitialized in this function [-Wuninitialized] printf ("x = %d\n", x);

9 Valgrind - Uninitialized Variables (4/7) wolf:~/lab6$ valgrind --tool=memcheck lab6 ==33083== Memcheck, a memory error detector ==33083== Use of uninitialised value of size 8 ==33083== at 0x4E8099B: _itoa_word (_itoa.c:179) ==33083== by 0x4E84636: vfprintf (vfprintf.c:1660) ==33083== by 0x4E8B3D8: printf (printf.c:33) ==33083== by 0x400548: main (lab6.c:4)

10 Valgrind - Uninitialized Variables (5/7) #include <stdio.h> int main() { char *s; char ch = *s; printf ("The first character of the string is: %c\n", ch); return 0; } What s the problem in this code snippet?

11 Valgrind - Uninitialized Variables (6/7) #include <stdio.h> int main() { char *s; char ch = *s; printf ("The first character of the string is: %c\n", ch); return 0; } wolf:~/lab6$ gcc -Wall -g -std=c99 -o lab6 lab6.c lab6.c: In function 'main': lab6.c:4:7: warning: 's' is used uninitialized in this function [-Wuninitialized] char ch = *s;

12 Valgrind - Uninitialized Variables (7/7) wolf:~/lab6$ valgrind --tool=memcheck lab6 ==42849== Memcheck, a memory error detector ==42849== Use of uninitialised value of size 8 ==42849== at 0x400539: main (lab6.c:4) ==42849== ==42849== Invalid read of size 1 ==42849== at 0x400539: main (lab6.c:4) ==42849== Address 0x0 is not stack'd, malloc'd or (recently) free'd

13 Valgrind - Reading/Writing memory that has already been freed (1/3) #include <stdio.h> #include <stdlib.h> int main() { char *ptr = malloc(sizeof(char)); *ptr = A ; free(ptr); printf ("The pointer points to: %c\n", *ptr); return 0; } What s the problem in this code snippet?

14 Valgrind - Reading/Writing memory that has already been freed (2/3) #include <stdio.h> #include <stdlib.h> int main() { char *ptr = malloc(sizeof(char)); *ptr = A ; free(ptr); } printf ("The pointer points to: %c\n", *ptr); return 0; The code compiles without any errors or warnings!

15 Valgrind - Reading/Writing memory that has already been freed (3/3) wolf:~/lab6$ valgrind --tool=memcheck lab6 ==44888== Memcheck, a memory error detector ==44888== Invalid read of size 1 ==44888== at 0x4005EA: main (lab6.c:7) ==44888== Address 0x51fc040 is 0 bytes inside a block of size 1 free'd ==44888== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==44888== by 0x4005E5: main (lab6.c:6)

16 Valgrind - Reading/Writing memory after the end of a malloc d block (1/3) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; char ch = *(ptr + 1); printf ("The character equals to: %c\n", ch); free(ptr); return 0; What s the problem in this code snippet?

17 Valgrind - Reading/Writing memory after the end of a malloc d block (2/3) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; char ch = *(ptr+1); printf ("The character equals to: %c\n", ch); free(ptr); return 0; The code compiles without any errors or warnings!

18 Valgrind - Reading/Writing memory after the end of a malloc d block (3/3) wolf:~/lab6$ valgrind --tool=memcheck lab6 ==45461== Memcheck, a memory error detector ==45461== Invalid read of size 1 ==45461== at 0x4005DE: main (lab6.c:6) ==45461== Address 0x51fc041 is 0 bytes after a block of size 1 alloc'd ==45461== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==45461== by 0x4005CE: main (lab6.c:4)

19 Valgrind - Incorrect freeing of heap memory (1/3) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; printf ("The pointer points to: %c\n", *ptr); free(ptr); free(ptr); return 0; What s the problem in this code snippet?

20 Valgrind - Incorrect freeing of heap memory (2/3) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; printf ("The pointer points to: %c\n", *ptr); free(ptr); free(ptr); return 0; The code compiles without any errors or warnings!

21 Valgrind - Incorrect freeing of heap memory (3/3) wolf:~/lab6$ valgrind --tool=memcheck lab6 ==45699== Memcheck, a memory error detector ==45699== Invalid free() / delete / delete[] / realloc() ==45699== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==45699== by 0x40060C: main (lab6.c:8) ==45699== Address 0x51fc040 is 0 bytes inside a block of size 1 free'd ==45699== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==45699== by 0x400600: main (lab6.c:7)

22 Valgrind - Memory Leaks (1/4) It keeps track of all heap blocks allocated by a program. Specify the --leak-check=full flag on the Valgrind command line. For each block, there are four possible leaks: "Still reachable": The block pointer exist, but it was never freed. "Definitely lost": No pointers to the block can be found. "Indirectly lost": The block is lost, because all the blocks that point to it are lost. "Possibly lost".

23 Valgrind - Memory Leaks (2/4) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; printf ("The pointer points to: %c\n", *ptr); return 0; What s the problem in this code snippet?

24 Valgrind - Memory Leaks (3/4) #include <stdio.h> #include <stdlib.h> int main() { } char *ptr = malloc(sizeof(char)); *ptr = A ; printf ("The pointer points to: %c\n", *ptr); /* free(ptr); */ return 0; The code compiles without any errors or warnings!

25 Valgrind - Memory Leaks (4/4) wolf:~/lab6$ valgrind --tool=memcheck --leak-check=full lab6 == == Memcheck, a memory error detector ==46503== in use at exit: 1 bytes in 1 blocks ==46503== total heap usage: 1 allocs, 0 frees, 1 bytes allocated ==46503== 1 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==46503== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==46503== by 0x40058E: main (lab6.c:4)

Building Embedded Systems

All Rights Reserved. The contents of this document cannot be reproduced without prior permission of the authors. Building Embedded Systems Chapter 5: Maintenance and Debugging Andreas Knirsch andreas.knirsch@h-da.de